Corrosion inhibited hydraulic fluids



U.S. Cl. 25278 16 Claims ABSTRACT OF THE DISCLOSURE A novel hydraulic fluid composition consisting essentially of an organic phosphate, e.g., dialkyl phenyl phosphates, alkyl diaryl phosphates, triaryl phosphates and trialkyl phosphates, and a small amount of an amine inhibitor additive which composition is non-corrosive to the metals, copper, cadmium plated iron and magnesium. Process for reducing corrosion of the above named metals by the above named organic phosphates by incorporating therein small amounts of an amine inhibitor additive. Thus, dibutyl phenyl phosphate and 0.5% of N,N',N", N"'-tetra-salicylidene-tetra(amino-methyl) methane was noncorrosive to copper, cadmium plated iron and magnesium over extensive periods at elevated temperatures.

This invention relates to compositions and to hydraulic fluids containing phosphates, and is particularly concerned with improved phosphate-containing compositions and hydraulic fluids which are inhibited against corrosion of certain metals.

Improved hydraulic fluids have been developed which are particularly useful in hydraulic systems of aircraft. Certain hydraulic fluids of this type have been designed especially for operation at high temperatures in the hydraulic systems of modern aircraft, especially supersonic aircraft. Hydraulic fluids of this type generally contain as their essential component an organic phosphate. Examples of such improved hydraulic fluids are described in US. Patent 2,903,428 to Moreton, the hydraulic fluids described in this patent containing as a base component a dialkyl phenyl phosphate and a minor proportion of a polyalkyl methacrylate, as described in detail therein.

The improved hydraulic fluids such as those described in US. 2,903,428 have many advantages over prior art hydraulic fluids employed in hydraulic systems of aircraft. These advantages include excellent lubricating properties, high thermal and chemical stability, high fire resistance, and operability over a wide liquid range and particularly at elevated temperatures of the order of about 250 F. and above. Although these improved phosphate-containing hydraulic fluids are suitable for use with and are substantially noncorrosive to many metals and alloys such as aluminum and iron, including certain types of steels, such fluids, particularly at the elevated temperatures of operation of the order of about 250 F., cause corrosion of certain metals such as copper and cadmium plated steels, often used for construction of hydraulic system components, e.g., pumps and the like, employed in hydraulic systems of aircraft.

The development of suitable inhibitors against attack of the above described phosphate-containing compositions or hydraulic fluids on metals such as copper, and which are compatible and suitable for use with such organic phosphates, and particularly the hydraulic fluids containing such phosphates as an essential component, without adversely affecting the properties of the hydraulic fluids, has been complex. The difficulties in developing suitable additives for this purpose, and particularly for use with such hydraulic fluids, are the requirements that such additives must be compatible with or soluble in such phosphate-containing compositions or hydraulic fluids over a broad temperature range, the additives should not cause undesirable gelation of the compositions tending, for example, to transform such compositions or fluids into a thixotropic material, and should not have any adverse side effects or side reactions on the components of these compositions, particularly the organic phosphate. Also, the inhibitor should not operate to increase corrosion on the metals which are not normally affected by the phosphate-containing material, and also the volatility of the additive should be low enough so that the inhibitor is not volatilized at the higher temperatures of operation. Further, for preferred use in the above described hydraulic fluids which are intended for operation at high temperatures in the range of about 250 to about 300 F., and above, it is necessary that such additives have good thermal stability at temperatures of at least about 300 F., and fire resistance.

I have found that by addition of a small or minor amount of the amino compound N,N',N",N"'-tetra-salicylidene-tetra(amino-methyl)methane having the formula to an organic phosphate, particularly phosphate esters of the type employed in improved hydraulic fluids, and as hereinafter further defined, the resulting phosphate material or composition containing the same, unexpectedly exhibits substantially reduced corrosion on copper, cadmium plated iron, particularly cadmium plated low carbon steels, and magnesium, at elevated temperatures of the order of about 250 F. to about 300 F, as compared to the phosphate or phosphate-containing composition in the absence of such additive. The invention principles are particularly applicable to the phosphatecontaining mixtures forming the hydraulic fluids described in the above patent, preferably employing a dialkyl phenyl phosphate as a major component, and which may include a minor proportion of a suitable viscosity index improver as hereinafter described. Of particular significance, such corrosion inhibition is attained substantially without adversely affecting the properties of the phosphate-containing material and particularly the important properties of the above noted hydraulic fluids, in the absence of such additive.

The above noted amino additive is marketed by E. I. du Pont de Nemours and Co., Inc. under the designation PCA 155.

The term iron employed in the specification and claims, unless otherwise indicated, is intended to denote and include iron and steels, particularly low carbon steels.

The organic phosphates and hydraulic fluids containing the same which are inhibited according to the invention principles include dialkyl aryl phosphates, alkyl diaryl phosphates, trialkyl phosphates and triaryl phosphates.

The above noted inhibitor additive of the invention is particularly eifective for preventing corrosion by dialkyl aryl phosphates or hydraulic fluids containing the same on metals such as copper and cadmium plated steel at elevated temperatures of the order of about 250 F. and above. These phosphate esters are preferably dialkyl phenyl phosphates in which the number of carbon atoms in each of the alkyl groups is from 4 to 8 and the total number of carbon atoms in the two alkyl groups is from 8 to 12. The alkyl groups may be straight or branched chain. For example, the following phosphates may be used: dibutyl phenyl, phosphate, butyl amyl phenyl phosphate, butyl hexyl phenyl phosphate, butyl heptyl phenyl phosphate, butyl octyl phenyl phosphate, diamyl phenyl phosphate, amyl hexyl phenyl phosphate, amyl heptyl phenyl phosphate, and dihexyl phenyl phosphate.

In accordance with the invention principles, the inhibitor additive hereof can be effectively incorporated in alkyl diaryl phosphates, or hydraulic fluids containing the same, in which the aryl radicals of such phosphates may have from 6 to 8 carbon atoms and may be phenyl, cresyl or xylyl, and the alkyl radical may have from about 4 to 12 carbon atoms. Examples of the alkyl diaryl phosphates include butyl diphenyl, amyl diphenyl, hexyl diphenyl, heptyl diphenyl, octyl diphenyl, 6-methy1 heptyl diphenyl, butyl phenyl cresyl, amyl phenyl xylyl, and butyl dicresyl phosphates.

Also effectively inhibited according to the invention principles are the triaryl phosphates or hydraulic fluids containing the same in which the aryl radicals of such phosphates have from 6 to 8 carbon atoms, that is, may

be phenyl, cresyl or xylyl, and in which the total number of carbon atoms in all three of the aryl radicals is from 19 to 24, that is, in which the three radicals include at least one cresyl or xylyl radical. Examples of such phosphates include tricresyl, trixylyl, phenyl dicresyl, and cresyl diphenyl phosphates. The triaryl phosphates have high fire resistance properties.

Trialkyl phosphates or hydraulic fluids containing the same may also be inhibited according to the invention principles, such phosphates having alkyl groups which are either straight chain or branched chain with from about 4 to about 9 carbon atoms, such as n-butyl, n-amyl and n-hexyl, particularly tri-n-butyl phosphate, tri(2- ethyl hexyl) phosphate and triisononyl phosphate, the straight chain alkyl groups preferably containing from 4 to 6 carbon atoms.

If desired, a single one of the above defined phosphates can be employed in the hydraulic fluid or a mixture of such phosphates can be employed. Thus, for example, mixtures of an above noted triaryl or alkyl diaryl phosphate, with an above noted trialkyl phosphate, e.g., a mixture of tricresyl phosphate and tri(2-ethyl hexyl) phosphate, can be utilized.

The above noted organic phosphates and hydraulic fluids containing the same are disclosed in Patents 2,566,623; 2,834,733; 2,903,428; and 3,136,726. The disclosures of such patents are incorporated herein by reference.

In compounding hydraulic fluids containing organic phosphate or phosphate esters as described above, it is often preferred to incorporate a compatible viscosity improving or thickening agent with the organic phosphate base stock or essential component, in order to provide a hydraulic fluid having proper viscosity characteristics at operating temperatures over a wide range from as low as about -60 F. up to about 250 to 300 F., and providing a minimum increase in viscosity at low temperature and a high viscosity index. Thus, such viscosity improver should be effective to increase the viscosity index of the phosphate-containing hydraulic fluid to at least 75, e.g., in the range of about 75 to about 150, with the resulting fluid containing the thickener having a viscosity at low temperature down to about -40 P. which is not in excess of about 14,000 centistokes. The viscosity improver should also have high thermal stability at temperature of at least 300 F., and provide a hydraulic fluid having high fire resistance. Examples of suitable thickeners for this purpose include the polyalkyl methacrylates, the polyalkylene glycols, and the polyurethanes, in the order of preference named.

The preferred polyalkyl methacrylates employed as viscosity improvers for the above organic phosphates have alkyl groups which can contain from 2 to 18 carbon atoms and have a molecular weight within the range of about 2,000 to about 20,000 and an average molecular weight within the range of about 3,000 to about 15,000. The particular polyalkyl methacrylate employed in conjunction with an organic phosphate is chosen for best compatibility with such. phosphate. Thus, where such viscosity improver is added to a dialkyl phenyl phosphate,

polyalkyl methacrylates are generally chosen in which the alkyl group has from 4 to 8 carbon atoms and an average molecular weight in the range from about 6,000 to about 12,000. For example, it has been found that poly butyl, poly amyl, poly hexyl, and poly octyl methacrylates may be used with butyl Z-ethylhexyl phenyl phosphate. When employing alkyl diaryl or triaryl phosphates together with the polyalkyl methacrylate viscosity improver, the alkyl groups of such methacrylate can contain, for example, from about 6 to about 12 carbon atoms.

Another suitable type of viscosity improver or thickener are the high molecular weight polyalkylene glycols. Suitable materials of this type are those in which the alkylene groups contain from 2 to 3 carbon atoms. Thus, specific examples of such polyalkylene glycols are the high molecular weight polypropylene glycols, polyisopropylene glycols, and copolymers such as the ethylene glycol, isopropylene glycol copolymer. Such high molecular weight polyalkylene glycols are characterized by having a range of viscosity of about 1,400 to about 23,000 centistokes at 100 F. Also suitable as viscosity improvers or thickeners are the urethane polymer liquids. These may be urethane polyether or urethane polyester materials.

Usually a minor proportion, and generally from about 0.2 to about 12% by weight of the viscosity index improver, e.g., polyalkyl methacrylate, based on the weight of the total composition, produces the desired effectiveness in the organic phosphate or the hydraulic fluid containing the same. In preferred practice a proportion of about 1% to about 10% by weight of the viscosity index improver is employed.

For increasing the fire resistance of the above described organic phosphates and particularly the alkyl diaryl phosphates and triaryl phosphates described above, and hydraulic fluids containing the same, there can be incorporated with the organic phosphate a chlorinated biphenyl, e.g., as described in U.S. Patent 3,136,726. Suitable chlorinated biphenyls for this purpose are those having from about 40 to about 55% combined chlorine such as the tri-, tetraand penta-chlorobiphenyls. When incorporated with such organic phosphates, e. g., in hydraulic fluids containing a triaryl phosphate or an alkyl diaryl phosphate, from about 20 to about as high as of chlorinated biphenyl by weight of the hydraulic fluid can be employed.

Improved fire resistance of hydraulic fluids containing an organic phosphate base stock as described above, and particularly an alkyl diaryl phosphate or a triaryl phosphate, can also be achieved by incorporating therein a chlorinated diphenyl ether, e.g., as described in my copending application Ser. No. 408,753 filed Nov. 4, 1964, now Patent No. 3,352,784. Suitable chlorinated diphenyl ethers which can be employed can include from about 1 to about 10 chlorine atoms in the molecule, generally from about 1 to about 6 chlorine atoms per molecule. Mixtures of these chlorinated diphenyl ethers can also be employed. A proportion of about 10% to as high as of the chlorinated diphenyl ether can be employed, by weight of the composition including such organic phosphate.

The amine additive N,N',N",N"-tetra-salicylidenetetra(amino-methyl)methane of the invention effectively inhibits corrosion of copper, cadmium plated iron and magnesium, by organic phosphates or hydraulic fluids containing such phosphates as essential components, and which also may contain one or more of the above noted additional components such as viscosity improvers, chlorinated biphenyls, and/or chlorinated diphenyl ethers.

The N, N, N, N tetra salicylidene tetra(aminomethyl) methane additive of the invention, in addition to possessing high inhibiting effectiveness of the attack of the above organic phosphates on copper, cadmium plated iron, and magnesium, particularly at elevated temperatures, also possesses substantially all of the characteristics noted above for proper suitability for use in hydraulie fluids containing such phosphates. Thus, the invention additive is compatible with such organic phosphates without adversely affecting the properties of such phosphates, is compatible with and soluble in such phosphates or phosphate-containing hydraulic fluids over a broad temperature range, does not cause gelation of such fluids or have any adverse side effects on the components of such hydraulic fluids, particularly the organic phosphates thereof. Also, the invention inhibitor does not cause any increase in corrosion on metals which are not normally affected by such phosphate-containing materials, and the volatility of the additive is sufliciently low that the inhibitor is not volatilized at temperatures of the order of 250 to 300F., and such additive has good thermal stability and fire resistance at temperatures of the order of about 250 to 300 F.

Only a small or minor amount of inhibitor additive of the invention is employed with a substantially or major proportion of the organic phosphate. Thus, the amount of the N,N',N",N"'-tetra-salicylidene-tetra(amino-methyl)methane additive incorporated into the organic phosphate-containing material, preferably the above described organic phosphates or hydraulic fluids containing the same, to achieve the above described inhibiting effectiveness, can range from about 0.001% to about 1%, preferably from about 0.001% to about 0.50%, by weight of the composition.

Although an amount of amine inhibitor additive greater than 1% can be employed, where such amount of additive is soluble in the base phosphate material, such larger amounts usually do not materially enhance the eflectiveness of the additive. Amounts of additive smaller than 0.001% by weight generally are of reduced eflectiveness in producing the desired inhibition of corrosion according to the invention.

The following are examples of practice according to the invention. In the examples below, the term additive is intended to denote N,N,N",N"'-tetra-salicylidenetetra (amino-methyl)methane.

EXAMPLE 1 Composition A is prepared by dissolving the below indicated amount of inhibitor in the indicated amount of phosphate.

Composition A Percent by wt.

Di n-butyl phenyl phosphate 99.5 Additive 0.5

Composition A above and a control consisting of di n-butyl phenyl phosphate alone in the absence of any inhibitor, are tested for corrosion on copper, cadmium plated iron in the form of a mild carbon steel meeting the requirements of Federal Specification QQ636, magnesium, aluminum and iron (low carbon steel), metals often used in the construction of hydraulic system for modern aircraft, in an oxidation and corrosion test conducted at 250 F. according to the standard test procedure Mil-H5606, pursuant to Federal Test Method Standard No. 791, Method 5308.4, employing a testing period of 168 hours.

The properties of the above two fluids, particularly their corrosive activity on the above metals, are set forth in Table I below.

The results of these tests show that at 250 F. substantial corrosion of the copper and cadmium plated steel samples occur in the control fluid not containing the invention inhibitor. Thus, the copper sample immersed in such fluid has a weight loss of 6.58 mg./cm. and the cadmium plated steel sample immersed in such fluid shows a weight loss of about 0.35 mg./cm. On the other hand, the tests show that the copper and cadmium plated steel samples contacted with fluid Composition A containing the invention inhibitor or additive only has a weight change of 0.10 and 0.03 mg./cm. respectively, and accordingly show substantially reduced corrosion following the above treatment of such samples in these fluids, as compared to the control. Although the corrosive activity of the control fluid on magnesium is relatively minor, 0.04 mg./cm. the corrosive activity of Comoposition A containing the invention inhibitor on magnesium is reduced even further, of the order of about one-half (0.02 mg./cm. as compared to that of the control.

Further, it is noted that the relative acidity of the control fluid following the test (1.80) is substantially greater than the acidity of Composition A (0.90) following such test, as seen in the table above, indicating substantially greater corrosive activity of the control as compared to Composition A.

It is noted that the aluminum and iron samples also tested at 250 F. with the two fluids noted above remain uncorroded in each of these fluids, showing that the invention inhibitor has substantially no adverse affect with respect to corrosion of such metals which are normally not corroded by the control fluid.

EXAMPLE 2 Tests are carried out to determine the corrosion activity of Compositions B and C below on the same metals as are tested in Example 1 above.

Composition B Percent by weight One series of tests of the eifectiveness of Compositions B and C on copper, cadmium plated steel, magnesium, iron and aluminum are carried out at 250 F., and another seriesof similar tests are carried out at 300 F.

In the tests both at 250 F. and 300 F. comparative results similar to those obtained in Example 1 are observed. Thus, Composition B, the control, containing no inhibitor according to the invention, causes substantial corrosion of the copper and cadmium plated steel samples TABLE I Kinematic viscosity in centistokes at 130 F.

Acidity (neutralization No.), mg. KOH/gm.

Eflect upon metals, weight change, mgJem.

Fl id Initial Final Initial Final Change Mg Fe Cd/Fe Cu Al Control 2. 96 3. 02 0. 00 1.80 +1. 0. 04 -0. 01 -0. 35 Composition A a. 21-2. 99 2. e9 0. 01 0. +0.89 -0. 02 --o. 01 -o. 03 l i3 -8.'

and minor corrosion of the magnesium sample, at 250 F., whereas Composition C, a hydraulic fluid containing phosphate, viscosity improver and invention inhibitor, substantially reduces corrosion of the copper and cadmium plated steel samples and further reduces corrosion of the magnesium sample at 250 F. At 300 F., substantial corrosion of the copper, cadmium plated steel and magnesium samples takes place when treated with the control Composition B, whereas substantially reduced corrosion takes place on all three of these samples at 300 F. with respect to the invention composition or hydraulic fluid C.

Similarly, as in the case of Example 1, Composition B, the control, and also the inhibited hydraulic fluid, Composition C, cause substantially no corrosion of the aluminum and iron samples at the above elevated temperatures.

Hence, it is apparent that the presence of the viscosity index improver, the polyalkyl methacrylate, in the phosphate-containing hydraulic fluids of Compositions B and C has substantially no effect on the function of the invention inhibitor in the inhibited hydraulic fluids of the invention for reducing corrosion on copper, cadmium plated iron and magnesium.

Further, the use of the smaller amount of the inhibitor or additive in this example as compared to the 0.5% of additive employed in Example 1, appears to produce approximately equivalent inhibition results as compared to the results of Example 1.

EXAMPLE 3 The following are additional examples of inhibited phosphate-containing compositions according to the invention.

Composition D Percent by wt.

Comparative results with respect to inhibition of the phosphate-containing Compositions D and E for preventing corrosion on copper, cadmium plated steel and magnesium at temperatures of the order of 250 to 300 F., are obtainable similar to the comparative results in Examples 1 and 2 above, employing the smaller amounts of additive in Compositions D and E.

EXAMPLE 4 Further additional examples of inhibited phosphatecontaining compositions according to the invention are set forth below.

COMPOSITION F Percent by wt.

G-methyl-phenyl diphenyl phosphate 99.9 Additive 0.1

8 COMPOSITION G 6-methyl-heptyl diphenyl phosphate 94.9 Polyalkyl methacrylate having alkyl groups from C to C and an average molecular weight of 8,000 to 10,000 5.0 Additive 0.1

100.0 COMPOSITION I-I Tricresyl phosphate 49.9 Tri-(Z-ethylhexyl) phosphate 49.9 Additive 0.2

100.0 COMPOSITION J Tricresyl phosphate 49.95 Chlorinated biphenyl (containing 48% combined chlorine) 49.95 Additive 0.1

100.00 COMPOSITION K Tricresyl phosphate 47.95 Chlorinated biphenyl (containing 48% combined chlorine) 47.95 Polyalkyl methacrylate having alkyl groups from C to C and an average molecular weight of 8,000 to 10,000 4.0 Additive 0.1

100.00 COMPOSITION L Butyl diphenyl phosphate 60 Chlorinated biphenyl (containing 54% combined chlorine) 39.7 Additive 0.3

100.0 COMPOSITION M Cresyl diphenyl phosphate 49.95 Trichlorodiphenyl ether 49.95 Additive 0.1

From the foregoing, it is seen that the invention provides phosphate-containing compositions and valuable phosphate-containing hydraulic fluids which are inhibited against attack of copper, cadmium plated iron, particularly cadmium plated low carbon steels, and magnesium, important metals of construction of components of aircraft hydraulic systems, and which metals are subject to substantial corrosion in the presence of the uninhibited phosphate-containing compositions and hydraulic fluids, especially at elevated temperatures.

While I have described particular embodiments of my invention for the purpose of illustration, it should be understood that various modifications and adaptations thereof may be made within the spirit of the invention within the scope of the appended claims.

I claim:

1. A composition consisting essentially of an organic phosphate selected. from the group consisting of dialkyl phenyl phosphates wherein the number of carbon atoms in each of the alkyl groups is from 4 to 8 and the total number of carbon atoms in the two alkyl groups is from 8 to 12, alkyl diaryl phosphates in which the alkyl radical contains from about 4 to about 12 carbon atoms and the aryl radicals are each selected from the group consisting of phenyl, cresyl and xylyl radicals, triaryl phosphates in which the aryl radicals are each selected from the' group consisting of phenyl, cresyland xylyl radicals and the total number of carbon atoms is from 19 to 24,

trialkyl phosphates the alkyl groups of which contain from about 4 to about 9 carbon atoms and from 0.001% to 1% of N,N,N",N"'-tetra-salicylidene-tetra(amino methyl) methane as additive.

2. A composition as defined in claim 1, wherein said additive is present in an amount ranging from about 0.001% to about 0.50% by weight.

3. A composition as defined in claim 1, wherein said organic phosphate is a dialkyl phenyl phosphate wherein the number of carbon atoms in each of the alkyl groups is from 4 to 8 and the total number of carbon atoms in the two alkyl groups is from 8 to 12.

4. A composition as defined in claim 1, wherein said organic phosphate is dibutyl phenyl phosphate.

5. A composition as defined in claim 1, wherein said organic phosphate is an alkyl diaryl phosphate in which the alkyl radical contains from about 4 to about 12 carbon atoms, and the aryl radicals have from 6 to 8 carbon atoms.

6. A composition as defined in claim 1, wherein said organic phosphate is a triaryl phosphate in which the aryl radicals have from 6 to 8 carbon atoms, and the total number of carbon atoms is from 19 to 24.

7. A composition as defined in claim 1, wherein said organic phosphate is a trialkyl phosphate, the alkyl groups of which contain from about 4 to about 9 carbon atoms.

8. A composition as defined in claim 1, and including from about 0.2% to about 12% of a viscosity index improver selected from the group consisting of polyalkyl methacrylates in which the alkyl group has from 2 to 18 carbon atoms and having a molecular weight within the range of about 2,000 to about 20,000 and an average molecular weight within the range of about 3,000 to about 15,000, polyalkylene glycols in which the alkylene groups contain from 2 to 3 carbon atoms and having a viscosity range of about 1,400 to about 23,000 centistokes at 100 F., and urethane polymer liquids.

9. A composition as defined in claim 1 and including from about 0.2% to about 12% of a polyalkyl methacrylate in which the alky group has from 2 to 18 carbon atoms and having a molecular weight within the range of about 2,000 to about 20,000 and an average molecular weight within the range of about 3,000 to about 15,000.

10. A fire resistant hydraulic fiuid composition as defined in claim 1, wherein said organic phosphate is a dialkyl phenyl phosphate wherein the number of carbon atoms in each of the alkyl groups is from 4 to 8 and the total number of carbon atoms in the two alkyl groups is from 8 to 12, wherein said additive is present in an amount ranging from about 0.001% to about 1% by weight, and including from about 0.2 to about 12% by weight of a polyalkyl methacrylate in which the alkyl group has from 4 to 8 carbon atoms, said methacrylate having an average molecular weight of about 6,000 to 10 about 12,000 and a molecular weight range from about 2,000 to about 20,000.

11. A fire resistant hydraulic fluid composition as defined in claim 10, wherein said phosphate is dibutyl phenyl phosphate, and said additive is present in an amount ranging from about 0.001% to about 0.50% by weight.

12. A fire resistant hydraulic fluid composition as defined in claim 1, and including a chlorinated biphenyl.

13. A composition as defined in claim 1, and including a chlorinated diphenyl ether.

14. A process for reducing the corrosion on a surface selected from the group consisting of copper, cadmium plated iron and magnesium, by an organic phosphate selected from the group consisting of dialkyl phenyl phosphates selected from the group consisting of dialkyl phenyl phosphates wherein the number of carbon atoms in each of the alkyl groups is from 4 to 8 and the total number of carbon atoms in the two alkyl groups is from 8 to 12, alkyl diaryl phosphates in which the alkyl radical contains from about 4 to about 12 atoms and the aryl radicals are each selected from the group consisting of phenyl, cresyl and Xylyl radicals, triaryl phosphates in which the aryl radicals are each selected from the group consisting of phenyl, cresyl and Xylyl radicals and the total number of carbon atoms is from 19 to 24, trialkyl phosphates the alkyl groups of which contain from about 4 to about 9 carbon atoms at elevated temperature, which comprises incorporating in said organic phosphate from 0.001 to 1% of N,N,N",N-tetra salicylidene tetra (amino-methyl) methane as additive.

15. A process as defined in claim 14, wherein said organic phosphate is a dialkyl phenyl phosphate wherein the number of carbon atoms in each of the alkyl groups is from 4 to 8 and the total number of carbon atoms in the two alkyl groups is from 8 to 12, and wherein said additive is present in an amunt ranging from about 0.001% to about 1% by weight.

16. A process as defined in claim 15, wherein said phosphate is dibutyl phenyl phosphate, and said additive is present in an amount ranging from about 0.001% to about 0.50% by weight.

References Cited UNITED STATES PATENTS 2,636,862 4/1953 Watson 252-78 3,282,847 11/1966 Schneider et al 252-78 3,352,780 1l/l967 Groslambert 25278 X LEON D. ROSDOL, Primary Examiner S. D. SCHWARTZ, Assistant Examiner US. Cl. X.R. 

